The effect of DNA demethylation on human immunodeficiency virus type-1 (H-1)IV expression was investigated in several cell lines infected with HIV-1 in vitro. In order to demethylate proviral DNA, 5-azacytidine (AZC) and its analog were employed. A persistently HIV-1-infected T-cell line, ACH2, produced a low level of HIV-1 in vitro. However, following exposure to AZC or its analog, a profound potentiation of viral expression occurred as assessed by syncytia formation and production of reverse transcriptase and p24 Gag protein. Alternation in the methylation pattern of proviral DNA in ACH2 was confirmed by Southern blot hybridization using methylation sensitive/insensitive isoschizomer restriction enzymes. AZC-induced potentiation of viral expression did not occur in any of high HIV-1-producers, chronically HIV-1 infected H9 cells, subacutely HIV-1-infected normal CD4+ T-cells, or subacutely infected lectin-activated peripheral blood mononuclear cells. Southern blot hybridization revealed that proviral DNA in these highly HIV-1-producing cells had already been extensively demethylated, and no altered DNA methylation pattern had occurred following AZC treatment. We also found that the inherent toxicity of AZC per se was, at least in part, associated with the observed enhancement of HIV-1 expression in ACH2 cells. The present data suggest that DNA methylation may play a role in regulating HIV-1 expression at least under certain circumstances. Further study is also warranted to test the possibility that certain therapeutics which exert toxicity to cells such as anticancer agents may affect the expression of HIV-1 in patients with HIV-1 infection and neoplasms.